C# WPF Desktop App - List - Threading problem - locking problem

Question

C# WPF Desktop App - List - Threading problem - locking problem

Markus Freitag 3,791

Hello,

Concecpt_05

I have a user interface where I fill a list.
At the same time, another process can update or delete the items.
It crashes when I add new items at the same time the process is deleting an item because it is processed. So it's finished.

The questions now, how can I make sure that when I add, the deletion of items is then delayed. How can I lock it?

Do you have sample code for me? Possible solutions? Thank you for your help in advance.
I use VS2019, C#, NET 4.72, Win10, Winforms or WPF Desktop App

RLWA32 49,551 Reputation points

2023-01-14T08:58:52.5033333+00:00

Typically a system-wide (i.e., named) mutex would be used to provide cross-process thread synchronization when exclusive access is needed for a shared resource.

https://learn.microsoft.com/en-us/dotnet/api/system.threading.mutex?view=net-7.0
Viorel 122.6K Reputation points

2023-01-14T09:35:23.9133333+00:00

What kind of lists do you mean and how is it shared between processes?
Markus Freitag 3,791 Reputation points

2023-01-14T15:30:34.7866667+00:00

Thanks!
It's one application, an exe.

I fill the list via the user interface, the working thread processes the list. I don't know how to lock this.

Same Exe, is a lock enough?

UserInterface
ListProcess.Add(..)

WorkerThread
ListProcess.Edit(..) or

ListProcess.Delete(..)

Is there an example for this use case?
Viorel 122.6K Reputation points

2023-01-14T17:28:52.9966667+00:00
I think that the lock statement can be used in some circumstances:

lock( ListProcess.SyncRoot ) { // ... add or remove items ... }

Buit it depends on specific details.
Markus Freitag 3,791 Reputation points

2023-01-15T15:26:44.2966667+00:00
Buit it depends on specific details.

Unfortunately it is not clear to me. I hope I can ask. UserInterface filled, worker thread updated or delete elements. With the user interface I have to work with dispatcher, do you know a sample?

Filling, updating, deleting must be done in one function? Am I correct?

So that it works correctly. The lock ensures that when filling is in progress, removal is waiting.

Is it correct that way?

public void ListProdcess(int mode) { lock( ListProcess.SyncRoot ) { switch (mode) { default: Exception break; case 1: // fill break; case 2: // modify break; case 3: // delete break; } } }
RLWA32 49,551 Reputation points

2023-01-15T15:45:42.2866667+00:00

Is the resource (i.e., List) that is shared by threads a UI element (e.g., control) or is it simply a collection that is updated by both the UI thread and a worker thread?
Markus Freitag 3,791 Reputation points

2023-01-15T15:52:31.5233333+00:00
List<int, Batch> ListProcess; public class Batch { int Quantity; string Name; string Supplier; int Age; DateTime ExpiryDate; }

It is a simple collection. I fill it in a UserInterface object later, so the operator can see it.
RLWA32 49,551 Reputation points

2023-01-15T15:58:47.63+00:00

It is a simple collection. I fill it in a UserInterface object later, so the operator can see it.

So it seems to me that as along as the UI thread and the worker thread lock the resource when they use it that there is no need to marshal updates from the worker thread to the UI thread.
Markus Freitag 3,791 Reputation points

2023-01-15T19:22:05.94+00:00

the UI thread and the worker thread lock the resource when they use it

Not clear what you mean.

Sorry, my mistake, is same list of course!

Batch Quantity is for example 100

I am processing the hundredth product, after that I have to delete it from the list.

Meanwhile the operator adds new batches.

How would you add that, how would you call that delete. What function, what does your function look like?

Can you make a sample for me?

Thanks!

Accepted answer

2 additional answers

Your answer

RLWA32 49,551 Reputation points

2023-01-14T08:58:52.5033333+00:00

Typically a system-wide (i.e., named) mutex would be used to provide cross-process thread synchronization when exclusive access is needed for a shared resource.

https://learn.microsoft.com/en-us/dotnet/api/system.threading.mutex?view=net-7.0
Viorel 122.6K Reputation points

2023-01-14T09:35:23.9133333+00:00

What kind of lists do you mean and how is it shared between processes?
Markus Freitag 3,791 Reputation points

2023-01-14T15:30:34.7866667+00:00

Thanks!
It's one application, an exe.

I fill the list via the user interface, the working thread processes the list. I don't know how to lock this.

Same Exe, is a lock enough?

UserInterface
ListProcess.Add(..)

WorkerThread
ListProcess.Edit(..) or

ListProcess.Delete(..)

Is there an example for this use case?
Viorel 122.6K Reputation points

2023-01-14T17:28:52.9966667+00:00

I think that the lock statement can be used in some circumstances:

lock( ListProcess.SyncRoot ) { // ... add or remove items ... }

Buit it depends on specific details.
Markus Freitag 3,791 Reputation points

2023-01-15T15:26:44.2966667+00:00

Buit it depends on specific details.

Unfortunately it is not clear to me. I hope I can ask. UserInterface filled, worker thread updated or delete elements. With the user interface I have to work with dispatcher, do you know a sample?

Filling, updating, deleting must be done in one function? Am I correct?

So that it works correctly. The lock ensures that when filling is in progress, removal is waiting.

Is it correct that way?

public void ListProdcess(int mode) { lock( ListProcess.SyncRoot ) { switch (mode) { default: Exception break; case 1: // fill break; case 2: // modify break; case 3: // delete break; } } }
RLWA32 49,551 Reputation points

2023-01-15T15:45:42.2866667+00:00

Is the resource (i.e., List) that is shared by threads a UI element (e.g., control) or is it simply a collection that is updated by both the UI thread and a worker thread?
Markus Freitag 3,791 Reputation points

2023-01-15T15:52:31.5233333+00:00

List<int, Batch> ListProcess; public class Batch { int Quantity; string Name; string Supplier; int Age; DateTime ExpiryDate; }

It is a simple collection. I fill it in a UserInterface object later, so the operator can see it.
RLWA32 49,551 Reputation points

2023-01-15T15:58:47.63+00:00

It is a simple collection. I fill it in a UserInterface object later, so the operator can see it.

So it seems to me that as along as the UI thread and the worker thread lock the resource when they use it that there is no need to marshal updates from the worker thread to the UI thread.
Markus Freitag 3,791 Reputation points

2023-01-15T19:22:05.94+00:00

the UI thread and the worker thread lock the resource when they use it

Not clear what you mean.

Sorry, my mistake, is same list of course!

Batch Quantity is for example 100

I am processing the hundredth product, after that I have to delete it from the list.

Meanwhile the operator adds new batches.

How would you add that, how would you call that delete. What function, what does your function look like?

Can you make a sample for me?

Thanks!

Answer 1

Bruce (SqlWork.com) 77,926 Volunteer Moderator

The UI controls are not thread safe and must be updated from the UI thread. The UI thread should be the only thread accessing the list control. For the background thread to call the ui thread use dispatcher

[https://learn.microsoft.com/en-us/dotnet/api/system.windows.threading.dispatcher.invoke?view=windowsdesktop-7.0

Markus Freitag 3,791 Reputation points

2023-01-15T15:28:42.6433333+00:00

Do you know a good complete example in C# WPF Desktop App? Thanks in advance.

Answer 2

محمود اليازجي 0

public void ListProdcess(int mode)
{
	lock( ListProcess.SyncRoot )
	{
		  switch (mode)
		  {
		  default:
			 Exception
			  break;
		  case 1: // fill
			 break;
		  case 2: // modify
			 break;
		   case 3: // delete
			 break;
		  }
	}
}

Answer 3

You could try to use bindingOperations.enableCollectionSyNChronization Method .

The documentation is pretty clear on this:

To use a collection on multiple threads, one of which is the UI thread that owns the ItemsControl, an application has the following responsibilities:

Choose a synchronization mechanism.
Synchronize all access from the application to the collection using that mechanism.
Call EnableCollectionSynchronization to inform WPF of the mechanism.
... You can lock the collection when modifying it from another thread.

On dispatcher (UI) thread:

_itemsLock = new object();
Items = new ObservableCollection<Item>();
BindingOperations.EnableCollectionSynchronization(Items, _itemsLock);

Then from another thread:

lock (_itemsLock)
{

// Once locked, you can manipulate the collection safely from another thread
Items.Add(new Item());
}

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C# WPF Desktop App - List - Threading problem - locking problem

2 additional answers

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